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atf8611

99 wr400.... what did i break??

6 posts in this topic

What did I do?? back in December, I started the bike after it had sat for about a month. I let it run for a few minutes, but it stalled out when I tried to give it gas. I did not imediatly try to restart it. about a week later, i tried to start it again, but it was no good (I now know that it was probably just a fouled plug). I tried using starting fluid, but that was no help. after a few kicks with the starting fluid, I pulled in the decompression lever and kicked through a few times to clear it out. after the 4th or 5th kick, the spark must have ignited the little bit of starting fluid, because it went BOOM and backfired real bad. after this happened, I suddenly dont have full compression like i should. I can push past TDC with my hand if that is any indication of my compression now. What do I do now? Should I start tearing it down? any idea of what I messed up or what i will find when i take it apart? Is it possible to start it now that it doesnt have full compression? I dont have good luck with yamaha. Thanks for any help.

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When some engines backfire or kickback they can sometimes break the woodruff key on the crank which allows the sprocket to turn on the crank and change the camshaft timing. I'm almost sure that the WR has a sprocket that is fixed to the crank permanently so I'm guessing maybe you have a loose timing chain that has slipped it's teeth. Whatever has happened, you need to check cam timing before going any further! If it's slipped slightly and does start it could cause more damage. If timing is ok, it looks as though you'll have to remove the head and check inside. There are plenty of threads on here for checking timing or go to www.thumperfaq.com for tips and photos. Hope this is of some help to you.

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When some engines backfire or kickback they can sometimes break the woodruff key on the crank which allows the sprocket to turn on the crank and change the camshaft timing. I'm almost sure that the WR has a sprocket that is fixed to the crank permanently so I'm guessing maybe you have a loose timing chain that has slipped it's teeth. Whatever has happened, you need to check cam timing before going any further! If it's slipped slightly and does start it could cause more damage. If timing is ok, it looks as though you'll have to remove the head and check inside. There are plenty of threads on here for checking timing or go to www.thumperfaq.com for tips and photos. Hope this is of some help to you.

Thanks for the info. I will look into that, as i should have time tonight to tear into it.

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well, the timing was off, but resetting it didnt help the compression issue. It almost sounds like the air is escaping through the intake, but im not 100% on that.

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If air is comming back thru the intake you have a valve problem.

Unless your cam timing is waaay off the intake valves should never let air from compression back thru the intake.

Pull the covers off and insure the valve positions relative to the crank positions.

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If air is comming back thru the intake you have a valve problem.

Unless your cam timing is waaay off the intake valves should never let air from compression back thru the intake.

Pull the covers off and insure the valve positions relative to the crank positions.

im not 100% sure thats where its leaking from, but it definatly sounds like it. I also reset the timing to the correct marks and that didnt fix it. i guess i will have to pull the head and check the valves and piston.

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